Why does the sky appear dark and black to an astronaut instead of blue?

Sushil went to an eye specialist for a check-up. The doctor prescribed him to use spectacle lenses of $+0.5 \text{ D}$ power.
$(a)$ Name the defect of vision he is suffering from.
$(b)$ Find the focal length of the spectacle lens.

What is the range of clear vision for a person with normal vision?

$(a)$ Explain why the colour of a clear sky appears blue during the day with the help of a diagram.
$(b)$ Explain why the Sun looks reddish at the time of sunrise and sunset.

$(a)$ State two main causes of a person developing near-sightedness (myopia). With the help of a ray diagram, suggest how this disability can be corrected.
$(b)$ The far point of a myopic person is $150 \ cm$ in front of the eye. Calculate the focal length and power of the lens required to enable him to see distant objects clearly.

Which scientist first explained the dispersion of light?